If I wanted to know what percent of the MEPS population was taking lisinopril for instance, is that something I could extract from the database? Or does it only provide me with the number of Rx medications a person is taking without providing the name of those medications.

It appears that IPUMS MEPS just has a count of all prescribed medications purchased during the year (RXPRMEDSNO). For specific medications, you may want to look into the prescribed medicine files provided by AHRQ, which are available here.